Adaptive-focus liquid lens has been used over the years for eyeglasses, cameras, projectors, cell phone as well as other machine vision applications. Based on the operating mechanisms and device structures, liquid lenses can be classified into three types. The first type is the liquid lens having a fixed volume of clear liquid which is sandwiched between a rigid lens (or a planar substrate) and a thin transparent elastic membrane, as described in U.S. Pat. Nos. 5,138,494, 5,999,328 and 6,040,947 issued to Kurtin et al. and U.S. Pat. No. 5,526,067 issued to Cronin et al. By moving the periphery of the elastic membrane, the liquid inside the lens assembly is redistributed such that the curvature of the film is changed. The changed curvature of the liquid lens surface bounded by the elastic membrane can vary the optical power, also known as diopter, of the lens. The shortcoming of this kind of liquid lenses is the difficulty keeping the periphery of the elastic membrane parallel to that of the rigid lens or planar substrate during the distance change. Moreover, the operating system for tuning the focus is complicated.
The second type of liquid lens requires to pump liquid in and out the lens chamber in order to change the curvature of the elastic membrane surface. Such an operating mechanism is described in U.S. Pat. Nos. 5,684,637 and 6,715,876 issued to Floyd. These lenses operate by injecting or pumping liquid into the body of the lens, a complicated control system is usually needed; thus such lenses are bulky, expensive and sensitive to vibration.
The third type of lens focuses light based on the electrowetting mechanism; an applied voltage can change the curvature of the liquid lens, see for example, U.S. Pat. No. 6,369,954 to Berge et al and U.S. Pat. No. 6,665,127 to Bao et al. By applying an external voltage to the liquid, the surface profile of the liquid is tuned because of the contact angle change. As a result, the focal length of the liquid lens is varied. Usually the applied voltage is high and the lens system is complicated and expensive. Another drawback of the electrowetting lens is that it is accompanied by liquid evaporation.
In the present invention, a tunable-focus liquid lens is demonstrated based on the pressure induced liquid redistribution. The liquid lens is composed of a flat cell and a liquid. The rigid flat cell has two through holes but these two holes are not overlapped. For example, one is on the top substrate and the other is on the bottom. The two holes are sealed with elastic membranes. One membrane is adhered on the outer surface of a substrate and the other is adhered on the inner surface of another substrate. For the convenience of discussion, hereinafter we call the former hole as the reservoir hole and the latter as the lens hole. The liquid is fully filled in the cell chamber and sealed using glue in order to prevent any leakage. Initially the two membranes are flat, so no focusing effect takes place. Squeezing the outside membrane inward by an actuator or other electro-mechanical means will redistribute the liquid rapidly, thus the inside membrane will swell outward. Because of this membrane shape change, focusing effect occurs.
To overcome these problems, what is needed is a lens with large aperture, large focusing power, polarization independence, wide spectral range (form visible to infrared), high resolution, and relatively fast response time for real-time imaging. The lens aperture size, in principle, has no constraint, depending on the applications. For instances, it can be made in micron sizes for microlens array, millimeter for call phone zoom lens, to several centimeters for eyeglasses.